1. Field of the Invention
The present invention relates to a visual display system for displaying a video signal or image data in the form of a visible image, and to an exposure control apparatus for a video camera or the like.
2. Description of Related Art
In recent years, demand has been increasing for larger-area and/or higher-definition screens than the screens offered by conventional display devices such as cathode ray tubes (CRTs) or liquid crystal displays (LCDs). Such demand has been further increased by consumers' demand for large-screen televisions, coupled with the demand from many people who want to enjoy shows on a large screen or who need to view screens generated by a computer in conferences or the like.
LCDs are suitable for use with small-size computer systems and terminal devices, particularly laptop and portable computers; LCDs for such applications are constructed using liquid crystal cells, each individual cell corresponding to one pixel on the screen. LCDs are sensitive to temperature, are difficult to construct in large size, have a slow response, and require external illumination for viewing. The resolution of the LCD is limited by the complexity of its driving system, the result of which is that the size of the LCD generally increases with increasing resolution. This means an inevitable increase in the size of the projection optical system used, and hence an increase in the cost of a high-definition system. Another problem is that light passed through the LCD (or reflected from it) is polarized, as a result of which the sensation of brightness varies nonlinearly with the distance from the center of the field of view.
The most widely used display system is the CRT. However, the CRT has various shortcomings, the first one being the cost. The high cost is due to the difficulty involved in the production of large-size display tubes and the requirement of large volumes of raw materials (particularly glass) for construction. As a result, the display becomes very heavy, is not easy to transport, and therefore, is not suitable for use in a small-size display system. The resolution is also a problem with the CRT.
Another shortcoming of such conventional display systems is that they are essentially an analog type. Therefore, when displaying information stored in digital form in a computer, the information must first be converted into a form compatible with the analog raster scanning for displaying on the CRT.
Japanese Patent Application Laid-Open No. 3-40693 discloses a visual display system using a spatial light modulator to overcome the above enumerated shortcomings. The system disclosed therein will be described below as a prior art example that most closely relates to the present invention.
FIG. 1 shows the construction of a two-dimensional digital visual display system comprising an image generating system 501 and a display screen 502. Light emitted from a light source 510 is collected by a mirror 511 which reflects the light into a lens 512. The lenses 512, 513, and 514 together form a beam columnator which acts to collimate the incident light 509 into a columnar light 508, the light energy being thus intensified to enhance the overall efficiency of the system. A folding mirror 520 reflects the columnar light 508 through an optical path 507 into a spatial light modulator 515. The spatial light modulator 515 selectively redirects portions of the light, passed through the optical path 507, toward a magnifying lens 505 to form an image on the display screen 502. The spatial light modulator 515 used in this example is one generally known as a deformable mirror device.
The spatial light modulator 515 has a surface 516 onto which the light from the optical path 507 is projected. The surface 516 contains a plurality of switchable elements 517 which can be controlled so as to redirect the incident light toward the magnifying lens 505. When one element 517 is placed in a certain orientation, a portion of light passed through the optical path 507 is redirected into an optical path 506 toward the magnifying lens 505. The portion of the light is magnified or enlarged by the magnifying lens 505 and projected through an optical path 504 onto the display screen 502 to illuminate a pixel 503.
A computer 519 controls the operation of the spatial light modulator 515 via a bus 518 so that portions of the light from the optical path 507 are selectively redirected toward the display screen 502 to form an image thereon. The bus 518 carries necessary control signals and image information from the computer 519 to the spatial light modulator 515. The computer 519 is, for example, a digital signal processor.
FIG. 2 shows the configuration of an electronic optical apparatus used for the above described visual display system. As shown, a signal source 640, which is, for example, a TV tuner, is connected to an electronic optical system 644 via a bus 642. The bus 642 leads to an A/D converter 646. An analog signal is fed via the bus 642 to the A/D converter 646 for conversion into a digital code representing the chrominance and luminance information of each pixel of an image. The digital code from the A/D converter 646 is placed on a bus 648 for transfer to a buffer memory 650. The digital code is thus stored in the buffer memory 650. In a different mode, such digital code or information can be loaded into the buffer memory 650 from an external computer or graphics system via a bus 652.
The digital code or information represents an image to be displayed. The digital code stored in the buffer memory 650 is transferred to a central processing unit (CPU) 654 via a bus 658. The CPU 654 is connected to a video memory 660 via a bus 658. The CPU 654 decodes the video signal, including the chrominance and luminance signals, contained in the information transferred from the buffer memory 650. The CPU 654 is so programmed as to extract the image from the information and store the information, including the chrominance and luminance signals, into the video memory 660. The image is further processed by the CPU 654 through a command given via a bus 662 or under control of the program contained in the CPU 654. Thereafter, the processed image data is transferred to a shift register for input to an array in the space light modulator, the image data being loaded into row and column decoders. It is also possible to load image information into the video memory 660 from an external computer or graphics system via a bus 668.
The electronic optical system 644, together with a projection system 672, constitutes an image generating system 674, for which the image generating system 501 of FIG. 1 with the spatial light modulator 515 connected via the bus 518 to the video memory 660 can be substituted. The image stored in the video memory 660 is transferred to the projection system 672 via a bus 670 and is displayed through an optical path 676 on a display screen 678.
In the prior art visual display system, however, the spatial light modulator is a flat surface type wherein numerous mirrors are arranged in minute patterns on a semiconductor surface. The space light modulator of such construction having a plurality of controllable elements arranged on a flat surface has the problem of low yields in semiconductor fabrication due to a large silicon chip area, increasing the cost and reducing the reliability of the visual display system. Another problem is that addressing circuitry for the controllable elements of surface type and circuitry for light modulation, i.e., circuitry for driving the controllable elements, become large and complex, which is a limiting factor in the downsizing of the system. There is the further problem that the design freedom is limited in that the width-to-height ratio (aspect ratio) of the image forming surface is determined by the shape of the spatial light modulator unless an additional special optical system is provided.
FIG. 3 is a diagram illustrating the construction of a prior art diaphragm mechanism. When an actuator 705 is moved, a ring 706 is turned, which in turn moves an actuating pin 704 fixed on the ring 706 to move a diaphragm fan 701 over a lens 702 with a fixed pin 703 acting as the fulcrum, so that the size of the aperture area is varied.
FIG. 4 is a block diagram illustrating a prior art lens diaphragm control mechanism. The lens diaphragm control mechanism comprises: a lens 801; a diaphragm 802; an image pick-up element for converting the optical image into electrical signals; a preamplifier 804 for amplifying the output of the image pick-up element 803 to a suitable magnitude; a photometric circuit 805 for extracting, out of the video signal outputted from the preamplifier 804, the portions that correspond to the photometric area; a detection circuit 806; a diaphragm control circuit, 807 for feedback control of the diaphragm 802 to bring the output of the detection circuit 806 into agreement with a predetermined reference voltage; and a pulse generator 808 for controlling the photometric circuit 805 by using vertical and horizontal synchronizing pulses or the like.
Light passed through the lens 801 is focused onto the image pick-up element 803, the amount of light being suitably controlled by the diaphragm 802. The image pick-up element 803 converts the optical image focused thereon into an electrical signal, i.e. a video signal, which is then amplified by the preamplifier 804 to a magnitude suitable for later processing. The amplified video signal is fed to the photometric circuit 805, which is under control of the pulse generator 808, and only the portions of the video signal that correspond to the photometric area are supplied to the detection circuit 806. The detection circuit 806 integrates the input signal to produce a photometric signal of the level corresponding to the mean luminance of the photometric area of the video signal. The photometric signal is inputted to the diaphragm control circuit 807. The diaphragm control circuit 807 performs feedback control of the diaphragm 802 to bring this value into agreement with the predetermined reference voltage.
However, there are an infinite variety of objects that may be placed for imaging at the image pick-up position, and they cannot be limited to particular types. From some kinds of objects, good images can be obtained with a fixed photometric area, while from others, proper images cannot be obtained with the fixed photometric area. For example, in a backlight situation where the object of interest is positioned in the center of the scene against a background consisting mostly of high luminosity areas, the high luminosity areas will become dominant factors in the photometric area, causing the diaphragm to tend to close down for the object; the resulting image of the object will appear all black. Conversely, in an excessive frontlight situation where most of the background is dark, the diaphragm will tend to open up for the object because of the darkness of the background, causing everywhere in the final image to appear pale and washed out.
FIGS. 5(a) and 5(b) are schematic diagrams illustrating the construction of a prior art image projection system. In FIG. 5(a), a lens 910 is directed toward the object for imaging. At this time, a reflecting mirror 911 is positioned as shown by dotted line, so that the object is focused on an image pick-up element 912 by the lens 910. When it is desired to enlarge the image for projection, the system is rearranged as shown in FIG. 5(b). At this time, the reflecting mirror 911 is manipulated and placed in a horizontal position as shown by solid line. In this arrangement, the image on a liquid crystal display 913 illuminated by a light source 914 is enlarged through the lens 910 for projection. The above construction is disclosed in Japanese Patent Application Laid-Open No. 2-13071.
In the prior art image projection system of the above construction, since the system configuration must be rearranged between the imaging and image projection situations, it takes a lot of time and effort for the rearrangement. Furthermore, since a component in the optical path is moved, the focusing accuracy of the system lowers.
As described above, the prior art exposure control apparatus relies on a lot of mechanical actions for controlling the diaphragm, and therefore, has the problem of slow operating speed and low reliability. Furthermore, the prior art apparatus needs a large number of parts, and is complex and expensive in construction. Moreover, because of the construction of the apparatus which is incorporated in the lens system, exposure control can only be performed collectively on the entire light sensitive elements, and partial corrections of exposure cannot be performed. Furthermore, in the prior art, no exposure control apparatus has been available with additional functions other than its originally intended functions.